It is known that application of a periodic perturbation to a surface of a surface-acoustic-wave substrate provides a grating which functions as a surface-wave reflector having a frequency selecting property.
Prior art surface-wave reflectors of this type are a strip-type reflector having a metal or other film, group-type reflector having grooves on one surface thereof, ion-implanted reflector, etc.
The strip-type reflector may be an arrangement using a film of Au or other heavy metal to mainly utilize its mass load effect or utilize the electric field short-circuit effect (.DELTA.v/v effect) of the metal film or an arrangement using a relatively thick film of Al or other metal or an insulator to utilize the geometric perturbation effect of a ridge.
However, the use of the Au or other heavy metal film strip, although expensive, does not provide a large reflection ratio per one grating. The electric field short-circuit effect is not expected unless the material has a large electromechanical coupling coefficient k.sup.2. The geometric perturbation effect of a ridge is absolutely determined by the elastic properties of the ridge and the substrate, and the light-metal film normally made from Al does not exhibit a large reflection ratio.
The group-type reflector which provides a relatively large reflection ratio is used widely. However, this reflector is not effective unless using a material which facilitates an etching for providing grooves. The most popular etching is a dry etching such as reactive ion etching enabling an anisotropic etching.
The ion-implanted reflector has a relatively small perturbation and a relatively small reflection ratio.
Therefore, these surface-wave reflectors are not satisfactory in viewpoints of productivity and reflection property.
Most prior art resonators including a piezoelectric film use a ZnO or AlN (aluminum nitride) piezoelectric film has the following disadvantages among others:
(1) An electrical instability occurs upon application of a voltage;
(2) It is difficult to form a qualified film; PA1 (3) A protective film (SiO.sub.2) is required on a silicon single-crystalline substrate; PA1 (4) The propagation loss of a surface acoustic wave is large in high frequency ranges; and PA1 (5) It does not match a normal silicon IC process.
Therefore, an AlN film is preferred particularly as a piezoelectric film for circuit integration into a single chip.
As an example providing a resonator using an AlN film on a silicon single-crystalline substrate, there is a report (L. G. Piearce et al., Appln. Phys. Letl Vol 39(1981) Dec., Vo. 11, (New York, U.S.A.) which discloses application of an AlN film to a two-port resonator.
The resonator has an AlN/SiO.sub.2 /Si arrangement in which its grating reflector consists of four hundred short-circuited Au strips.
It is reported that the resonator exhibits properties of resonance frequency: 121.7 MHz, insertion loss: 27 dB and quality factor: 3370. These are not sufficient properties.
Thus it is very difficult to obtain a small-scaled, excellent-property resonator, using the prior art arrangements.